Complex and single sequence cDNA probes will be used to study the expression of nonpolyadenylated mRNAs during postnatal development of the mouse brain. These probes will also be used to ascertain the extent to which the poly(A)-mRNAs are regulated at the transcriptional versus posttranscriptional level. Investigations pertinent to the biogenic history of poly(A)-mRNA will also be initiated. Other experiments will be performed to ascertain whether most poly(A)-mRNAs in brain are specific to that organ.